I'll admit I haven't watched the episode of the clone wars show that featured the dark saber (or any of the series, for that matter) but from all the screenshots/pictures of it, it looks like some fancy black sword that happens to be glowing white, with a far more defined shape than a lightsaber. It does NOT look even remotely like a "simple" lightsaber with a black core. (wiki article says it was originally going to just be a regular vibrosword, but Lucas didn't want lightsabers to be able to be blocked by regular weapons that look like swords, so it was made special... 'course, Unless I'm missing something lightsabers are getting blocked by vibroswords all the time in TOR (and KOTOR, for that matter), as well as similar weapons in the books.

I do have to agree with many of the other posters. The black core lightsabers just look silly/out of place.

(P.S. In the clone war episode, does said darksaber every actually "extend"? or is it really just a black sword with a white glow outline?)

you said it yourself: "Originally intended to be a vibroblade, George Lucas decided that, to preserve the uniqueness of the Jedi weapon, a lightsaber should not be seen to be blocked by sword-like weapon that is not a lightsaber. With that in mind, Lucas and the show's crew conceived of an ancient precursor to the lightsaber which they named the darksaber"

Also, if you don't like the look of black cored lightsabers don't use one. This game is not intended only for you Lucas fanbois for whom everything has to be canonical or its bunk. There's a reason there's not a trivia quiz or a basement dweller virginity check you have to pass before you sign up. Get over it